Torque wrenches are tools which are used to establish screwed connections with bolts being preloaded with a predefined force. In doing so screwed connections are loaded with big or small preload forces. The increasing application of new construction materials such as magnesium, aluminum and synthetic material leads to a steadily increasing number of sensitive screwed connections.
The torque wrench consists of a wrench lever with a grip and a wrench head, wherein the wrench head often is formed as squared fitting and serves as coupling for other tool receptions. In this way different insert tools for turning bolts or nuts can be placed on the squared fitting. In the process of establishing screwed connections, a force is transferred from a user to the wrench lever and the wrench head generating torque forces acting on a bolt or nut. The torque wrench is provided with a measuring device for measuring the exerted torque. With further devices the measured torque can be displayed continuously or the reaching of a preset torque is indicated.
Torque wrenches come in different mechanical or electronic designs. One example belonging to the mechanical group are the so-called clicker torque wrenches, which buckle or break by reaching a desired torque and in this way avoid further inducing of a moment. The measurement of electronic and digitally displaying torque wrenches is for example achieved by a strain gauge attached to a bending or torsion rod. Strain gauges change their electric resistance at small deformations. They are glued to steal parts such like the bending or torsion rod by using a special adhesive. One example of the torsion rod is a rod with fixed supports at both ends and a spring in the middle. In this way pivoting around the axis of rotation of the torsion rod is possible, which can be measured by the strain gauge. The obtained measurement signal is displayed on a display. Preset torques can be checked permanently during the tightening process by means of LEDs or the display. At the same time this kind of torque wrench can store all measurements or releasing made in an internal measured data storage. This data storage can then be readout via an interface or printed directly with a connected printer at all times.
The measuring device of the torque wrench is normally subjected to common wear and tear symptoms. The torque wrench must be calibrated and, if necessary, adjusted from time to time with an operating device to check if a torque measured with the measuring device is the same as an exerted torque. For this purpose a carrier with a transducer and a grip holder is provided in the operating device. The head portion of the torque wrench is tightly coupled to the transducer, for example with the square fitting. The grip of the torque wrench is fixed by the grip holder. A torque is exerted on the head portion of the torque wrench by a deflecting mechanism. This torque is measured by the transducer und displayed on a display provided at the operating device, for example at the transducer. The torque displayed by the torque wrench or a preset torque of the torque wrench for a release can be tested by comparison with the torque displayed on the display.
With known operating devices for testing torque wrenches different deflecting mechanisms for the generation of a torque are used. In principal two ways are possible for generating a torque at the head portion of a torque wrench. The transducer is either rotatably fixed to the carrier and the grip holder is immoveable coupled to the carrier or, in the other way, the transducer is immoveable fixed to the carrier and the grip holder is moveably mounted to the carrier.
The procedure of a calibration is predetermined and subject to a specific standard, e.g. according to the German DIN standard. In the beginning of a measurement for testing a torque wrench five preloads at 100% of nominal torque of the torque wrench are accomplished. Then five loads at 20%, at 60% and at 100% of the nominal torque are carried out. Up to now the test torques for a torque wrench have to be set individually when using common operations devices. The electro motor transfers the force to the head of the torque wrench with a deflecting mechanism which in particular can be designed as a gearing. The torques of the torque wrench to be tested are detected by a transducer. On the one hand the examiner has to enter the required test torques and on the other hand he has to read the values measured by the transducer. This leads to a large expenditure of time for calibrating torque wrenches according to the required standard.